1. Field of the Invention
The present invention relates to an optically functional device to control light by utilizing polarization-inverted structures in a LiNbO3 single crystal substrate, which is useful in various fields of e.g. optical information processing, optical processing technology, optical communication technology and optical measurement using a laser beam, a single crystal substrate for such an optically functional device, and a method of using such a single crystal substrate for the optically functional device.
2. Discussion of Background
A lithium niobate (LiNbO3) single crystal (hereinafter sometimes referred to simply as xe2x80x9cLNxe2x80x9d) known as a typical ferroelectric single crystal, is used mainly as a substrate for a surface acoustic wave device. With respect to this crystal, a single crystal having a large diameter and a high compositional homogeneity can be supplied at a relatively low cost. Further, this crystal is transparent within a wide wavelength range of from visible region to infrared, and it is possible to invert the ferroelectric polarization even at room temperature by applying a high electric field at a level of a few tens kV/mm. Accordingly, it has attracted attention in recent years also as a substrate for various optically functional devices such as non-linear optical devices or electro-optical devices, using the polarization-inverted structures.
Particularly, in recent years, it is expected to have second harmonic wave generating (SHG) devices of waveguide type developed to convert a semiconductor laser having a near infrared wavelength to a blue light having a half wavelength by a non-linear effect. Among them, a frequency conversion device is most thoroughly studied which employs an element of a structure having the polarization of an inorganic ferroelectric single crystal such as LN periodically inverted, as a light source for high density recording/read out of optical disks. Such a frequency conversion device is based on a quasi phase matching (QPM) system, which is a system to carry out phase matching by compensating the difference between propagation constants of the fundamental wave and the higher harmonic wave with the periodic structures.
This system has many excellent characteristics such that high conversion efficiency can be obtained, collimation or diffraction limit condensation of the output light is easy, and useful materials or wavelengths are not particularly limited. As the periodic structures for QPM, structures having the sign of the SHG coefficient (the d33 coefficient) periodically inverted, are most effective to obtain high efficiency, and with a ferroelectric crystal, positive or negative of the d coefficient corresponds to the polarity of the ferroelectric polarization, whereby the technology to have the ferroelectric polarization domains periodically inverted, becomes important.
As a device employing this system, a parametric oscillation wave-shifting device by means of the QPM system, has been reported, wherein periodically inverted structures are formed by applying an electric field of about 21 kV/mm to a LN single crystal, as disclosed in a literature (L. E. Myers et al., Optics Letters, 21, p591, 1996). Further, a SHG laser has been reported wherein polarization-inverted structures are formed in a periodic cycle of 4.75 xcexcm in a LN single crystal having MgO added, by means of a corona discharge method, as disclosed in a literature (A. Harada et al., Optics Letters, 22, p805, 1997).
Further, with respect to an optical device utilizing an electro-optical effect, for example, according to a literature (M. Yamada et al., Appl. Phys. Lett., 69, p3659, 1996), attention has been drawn to a cylindrical lens, a beam scanner, a switch, and an optical device to form polarization-inverted structures of a lens or prism shape in a LN single crystal as a ferroelectric crystal by applying a high electric field to the crystal and to polarize a laser beam passed therethrough by utilizing the electro-optical effect, as new optical devices. The LN single crystal is also considered to be promising as a substrate material.
With respect to a frequency conversion device or an electro-optical device utilizing the polarization-inverted structures of a ferroelectric LN single crystal, so far reported, in each case, a commercially available LN single crystal having a congruent composition with no additive or with MgO added, has been employed as the substrate crystal, because the LN single crystal so far available has been limited to a crystal having a congruent composition grown by a Czochralski method which is capable of growing a crystal having a large diameter inexpensively, from the industrial viewpoint. With respect to the LN crystal, it is well known from the correlation diagram of temperature/compositional ratios (phase diagram) that the stoichiometric composition and the congruent composition do not agree with each other.
Only the congruent composition is a composition whereby the composition of the melt and the composition of the crystal will agree, and a crystal having a uniform composition can be grown over the entire crystal. Accordingly, the composition of the LN single crystal which is presently produced and used for various applications, is a congruent composition having a molar fraction of Li2O/(Nb2O5+Li2O) of about 0.485 (the molar ratio of Li/Nb being about 0.94). Accordingly, in the conventional LN single crystal having a congruent composition, the Nb component is excessive, whereby Nb ions as much as a few percent are substituted for Li ions (anti-site defects), and Li ion sites also have a few percent of vacancy defects. The influence of these defects may not be serious for an application to a surface acoustic wave device, but can not be neglected for an application to an optical device. Accordingly, it has been desired to develop a crystal having a composition close to the stoichiometric composition and having non-stoichiometric defects reduced, as a substrate to be used for optically functional devices.
As is apparent from the phase diagram, for example, in the case of a LN single crystal, a crystal having a composition close to the stoichiometric composition can be precipitated from a melt having a composition having a Li concentration higher than the stoichiometric composition. However, when it is attempted to grow a crystal having the stoichiometric composition by the Czochralski method which has been used as a means to produce a LN crystal having a large diameter in a large amount on an industrial scale, the excessive Li component will remain in a crucible along with the precipitation of the crystal, and the compositional ratio of Li/Nb in the melt will gradually change, whereby the compositional ratio in the melt will reach the eutectic point soon after the initiation of growing. Accordingly, the solidification ratio of the crystal is restricted to a level as low as about 10%, and the quality of the precipitated crystal has been so poor that it can not be used for an optically functional device.
The present inventors have previously invented a lithium tantalate single crystal close to the stoichiometric composition having a molar fraction of Li2O/(Nb2O5+Li2O) of from 0.495 to 0.50 (the molar ratio of Li/Nb being from about 0.98 to 1.00) and having the non-stoichiometric defect concentration of the congruent composition substantially reduced, as a novel substance different from the conventional commercially available LN crystal having a congruent composition, and have filed a Patent Application (JP-A-10-45497). Further, they have reported about this novel crystal in a literature as follows. Namely, as a means to develop a crystal of high quality by reducing such non-stoichiometric defects, the present inventors have proposed a method of growing a crystal while continuously supplying the material (hereinafter referred to simply as the double crucible method of continuously supplying the material), for example, in a literature (K. Kitamura et al., Journal of Crystal Growth, Vol. 116, 1992, p327-332, or K. Kitamura et al., Applied Physics, Vol. 65, No. 9, 1996, p931-935).
Specifically, for example, in growing a LN single crystal close to the stoichiometric composition, the molar fraction of Li2O/(Nb2O5+Li2O) in the melt for growing was adjusted to be from 0.56 to 0.60 i.e. the Li component being excessive, and the crucible was made to have a double structure, whereby it was possible to pull up from the inner crucible, a LN crystal close to the stoichiometric composition having a molar fraction of Li2O/(Nb2O5+Li2O) of from 0.498 to 0.502 (the molar ratio of Ni/Nb being from about 0.99 to 1.01). The weight of the crystal being pulled up was measured at all times to obtain the growth rate, and a starting material powder having the same stoichiometric composition as the crystal, was continuously supplied between the outer and inner crucibles at that rate. By employing this method, it was possible to grow a long crystal, and a crystal solidification ratio of 100% based on the amount of the material supplied, was realized.
Further, in a recent literature (K. Kitamura et al., Journal of Crystal Growth, Vol. 25, No. 3, 1998, pA4), the present inventors have reported that with the above-mentioned LN single crystal close to the stoichiometric composition (the molar ratio of Ni/Nb being from 0.98 to 1.0) with no additive, the applied electric field required for the polarization inversion is at a level of ⅕ of the electric field heretofore applied. Namely, they have reported that the presence of a few percent of non-stoichiometric defects (anti-site defects or vacancy defects) in the conventional crystal having a congruent composition, possibly increases the applied voltage required to form the periodic polarization structures or the optical characteristics which the LN crystal essentially has.
Further, in a recent literature (Y. Furukawa et al., Journal of Crystal Growth, Vol. 211, 2000, p230-236), the present inventors have reported that with a crystal having a composition close to the stoichiometric composition, the optical damage resistance can sufficiently be improved by an addition of e.g. Mg in an amount as small as about 1 mol %, which used to be required in an amount of at least 5 mol % to improve the optical damage resistance of the conventional crystal having a congruent composition. In such a case, Mg is substituted also at Li sites, and as the amount of Mg added, increases, the Li/Nb molar ratio becomes small as compared with the crystal having no additive, and the Li/Nb molar ratio of the obtained crystal is from 0.95 to 1.0. Although the difference in the molar fraction is small between LN of stoichiometric composition and LN of congruent composition, the crystal characteristics substantially differ as the composition becomes close to the stoichiometric composition. Particularly, the crystal having a Li/Nb molar ratio within a range of from 0.95 to 1.01, has optical characteristics substantially different from the conventional crystal having a congruent composition.
In order to form polarization-inverted structures on a substrate made of a ferroelectric single crystal and to realize an optically functional device utilizing the interaction between the electro-optical effect and the non-linear optical effect of light passed through the polarization-inverted portions, the most important technique is to prepare from a few to as many as several hundreds polarization-inverted structures of a few xcexcm to several tens xcexcm sizes uniformly and with high precision.
As a method for forming polarization inversion, an electron beam irradiation method or a voltage application method is well known and commonly used. Such an optically functional device is used by passing light through the polarization-inverted portions, and if there is an optical distortion or loss at the respective polarization-inverted boundaries, very substantial optical non-uniformity will result as the entire device, whereby a device with high efficiency can not be realized.
An optical distortion will be formed at the boundary of a polarization-inverted portion, whereby a change in the refractive index as large as 10xe2x88x923 to 10xe2x88x924 or larger will result. A serious problem has been pointed out in a literature (V. Gopalan et al., J. Appl. Phys., vol. 80, p.6104, 1996) such that the change in the refractive index brings about scattering of the laser beam passed therethrough, whereby the operation of the device will depart from the ideal condition, and the device efficiency will decrease.
It is reported, for example, in the above-mentioned literature (L. E. Myers et al., Optics Letters, 21, p591, 1996) that it is necessary to relax the optical distortion by heating the crystal at 120xc2x0 C. for one hour after forming polarization-inverted structures by applying an electric field of about 21 kV/mm to the LN single crystal.
Further, in the above-mentioned literature, (M. Yamada et al., Appl. Phys. Lett., 69, p3659, 1996), it is reported that heat treatment is necessary after formation of polarization inversion by application of an electric field, also for an optical device having polarization-inverted structures of a lens or prism shape formed in a crystal by applying a high voltage to a LN single crystal as a ferroelectric crystal. It is reported that in this case, it is essential to heat the crystal substrate at 500xc2x0 C. in atmospheric air and to carry out the heat treatment for 5 hours in order to remove optical distortions at the polarization-inverted portions.
In a conventional voltage application method, it is common that a LN single crystal having a congruent composition of z-cut is employed, and a periodic electrode is provided on one side of the crystal, and a uniform electrode is provided on the other side, and the sample is maintained at room temperature or heated to a level of about 200xc2x0 C., and a pulse voltage is applied through the electrodes to invert the polarization of the portion immediately beneath the periodic electrode towards the z-axis direction. In the case of a conventional LN single crystal having a congruent composition, an applied voltage as high as at least 21 kV/mm is required for the polarization inversion.
Such a polarization inversion technique is to forcibly change the polarization i.e. the positions of Nb or Li ions in the crystal, at a temperature of not higher than the Curie temperature. With respect to the LN single crystal, it is known that the high voltage required for the polarization inversion may not necessarily be the direct cause for an optical distortion.
Namely, in the above-mentioned literature (A. Harada et al., Optics Letters, 22, p805, 1997), it is reported that with a LN single crystal having a congruent composition having 5 mol % of MgO added, the voltage required for the polarization inversion can be reduced to a level of about ⅕ of the voltage required for a usual congruent composition, but, even if this material is employed, heating at a temperature of 500xc2x0 C. for three hours is required to remove an optical distortion in a case where a SHG laser having polarization-inverted structures formed in a periodic cycle of 4.75 xcexcm in a LN single crystal having MgO added, by means of a corona discharge method, is to be prepared.
When the polarization-inverted boundaries of such a device having polarization-inverted structures formed in a substrate made of a conventional LN crystal having a congruent composition, were inspected by a polarization microscope, large optical distortions were observed at all of the polarization-inverted boundaries as shown in FIG. 1(a). Further, when the working laser beam was passed across the polarization-inverted portions, a very large propagation loss at a level of from a few percent to a few tens percent was observed. Formation of such optical distortions at the polarization-inverted boundaries, not only creates a problem of a large propagation loss but also makes it necessary to provide an extra heat treatment step for the preparation of an optically functional device to relax the optical distortions.
Further, another serious problem is that during the heat treatment to remove distortions, a pyroelectric effect is likely to form at polarization-inverted portions of a few xcexcm size once formed by e.g. an electric field application method at portions of a single polarization substrate, whereby the crystal is likely to be broken, or the sizes or positions of the polarization-inverted portions are likely to change although very slightly. Such a change creates a serious problem for the preparation of a device with high efficiency and good reproducibility.
The present inventors have continuously studied the characteristics of the LN single crystals as ferroelectric single crystals in order to solve the above-mentioned problems of the prior art and have found that with a LN single crystal having a composition close to the stoichiometric composition, even when polarization inversion is formed, optical distortions or propagation losses of light at the polarization-inverted boundaries are very small, and by using this single crystal as the substrate, it is possible to prepare an optically functional device having polarization-inverted structures, which has excellent properties.
Namely, the present invention provides an optically functional device comprising a ferroelectric single crystal substrate and polarization-inverted structures formed at portions of the substrate at a temperature of not higher than the Curie temperature by an electron beam scanning irradiation method or a voltage application method and designed to control light passed through the polarization-inverted portions, wherein a LiNbO3 crystal having a molar ratio of Li/Nb within a range of from 0.95 to 1.01, is used as the substrate, so that the propagation loss of light passed through the polarization-inverted portions immediately after formation of the polarization-inverted structures, is not more than 2%.
Further, the present invention provides an optically functional device comprising a ferroelectric single crystal substrate and polarization-inverted structures formed at portions of the substrate at a temperature of not higher than the Curie temperature by an electron beam scanning irradiation method or a voltage application method and designed to control light passed through the polarization-inverted portions, wherein a LiNbO3 crystal having a molar ratio of Li/Nb within a range of from 0.95 to 1.01, is used as the substrate, so that the change in the refractive index of the polarization-inverted boundaries is not more than 1xc3x9710xe2x88x924 without a heating step to remove an optical distortion at the polarization-inverted boundaries due to the directional inversion of spontaneous polarization in the ferroelectric crystal.
Further, the ferroelectric single crystal substrate to be used in the above optically functional devices is made of a LiNbO3 crystal having a molar ratio of Li/Ta within a range of from 0.95 to 1.00 and containing from 0.1 to 4.8 mol % of at least one element selected from the group consisting of Mg, Zn, Sc and In, as doped.
Still further, the present invention provides a single crystal substrate for an optically functional device to control light passed through a polarization-inverted portions, which is a ferroelectric single crystal substrate having polarization-inverted structures formed at a temperature of not higher than the Curie temperature by an electron beam scanning irradiation method or a voltage application method, wherein a LiNbO3 crystal having a molar ratio of Li/Nb within a range of from 0.95 to 1.01, is used as the substrate, so that a propagation loss of light of not more than 2% and an optical distortion of not more than 1xc3x9710xe2x88x926 are obtainable without a heat treatment.
The present invention further provides a method for improving the frequency conversion efficiency of an optically functional device, wherein a LiNbO3 crystal having a molar ratio of Li/Nb within a range of from 0.95 to 1.01, is used as a substrate for the optically functional device to carry out the frequency conversion of a laser entered into a single crystal having periodically inverted polarization structures by means of a nonlinear optical effect.
Furthermore, the present invention provides a method for improving the driving efficiency of an optically functional device, wherein a LiNbO3 crystal having a molar ratio of Li/Nb within a range of from 0.95 to 1.01, is used as a substrate for the optically functional device to control polarization or condensing of a laser beam entered into a single crystal having polarization structures inverted in a prism or lens shape by means of an electro-optical effect.